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A002011
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a(n) = 4*(2n+1)!/n!^2.
(Formerly M3598 N1458)
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6
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4, 24, 120, 560, 2520, 11088, 48048, 205920, 875160, 3695120, 15519504, 64899744, 270415600, 1123264800, 4653525600, 19234572480, 79342611480, 326704870800, 1343120024400, 5513861152800, 22606830726480, 92580354403680, 378737813469600
(list;
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listen;
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OFFSET
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0,1
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REFERENCES
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R. C. Mullin, E. Nemeth and P. J. Schellenberg, The enumeration of almost cubic maps, pp. 281-295 in Proceedings of the Louisiana Conference on Combinatorics, Graph Theory and Computer Science. Vol. 1, edited R. C. Mullin et al., 1970.
N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
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LINKS
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FORMULA
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G.f.: 4*(1-4x)^(-3/2).
a(n) = 1/J(n) where J(n) = Integral_{t=0..Pi/4} (cos(t)^2 - 1/2)^(2n+1). - Benoit Cloitre, Oct 17 2006
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MAPLE
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MATHEMATICA
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Table[4*(2*n + 1)!/n!^2, {n, 0, 20}] (* T. D. Noe, Aug 30 2012 *)
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PROG
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(PARI) a(n)=if(n<0, 0, 4*(2*n+1)!/n!^2)
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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Simpler description from Travis Kowalski (tkowalski(AT)coloradocollege.edu), Mar 20 2003
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STATUS
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approved
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